The present invention relates to improved abrasion resistant coatings, particularly for application to polymeric materials and metals. In particular, the present invention relates to inorganic/organic hybrid network coating materials produced by a sol-gel process, where the organic component is a silane-functionalized low molecular weight organic compound and the inorganic component is a metal alkoxide or a mixture of two or more metal alkoxides. The coatings of the present invention, in embodiments, provide improved optical abrasion resistance, improved hot-wet resistance, and improved UV resistance.
Transparent polymeric materials have been widely used in all aspects of daily life, and their use is steadily increasing. For example, transparent polymeric materials have found widespread use in windows for buildings and aircraft, glazing for automobiles, in household products such as glasses, and in optical lenses for a variety of applications. Unfortunately, the use of such transparent polymeric materials has been greatly limited and hindered by the non-abrasive resistant surfaces. In particular, the materials generally have a very low abrasion resistance, thereby making them susceptible to scratching, pitting, and dulling, thus reducing the transparency of the material.
Similarly, metals have always found widespread use in a wide variety of applications. In particular, aluminum has found widespread use for a variety of commercial and household items. However, as with the transparent polymeric materials, it is desired that the metal products have an increased abrasion resistance so that their physical and structural qualities and surface features can be maintained.
One solution to the non-abrasion resistance problem has been to coat the materials with a protective coating. Such a protective coating should have a higher abrasion resistance than the underlying substrate, thereby increasing the abrasion resistance of the material.
Various types of protective coating materials have been used to improve the abrasion resistance and hardness of materials such as transparent polymer material surfaces, such as polycarbonate resin. For example, Japan Kokai Tokkyo Koho JP 63 48,364 to Ishigaki et al. improved the scratch resistance of polymethylmethacrylate by utilizing silane compounds as coating materials. Japan Kokai Tokkyo Koho JP 63 152,675 to Komatsu et al. used a mixed inorganic composition of zirconate-silicate with a SnO.sub.2 sol as an antistatic and scratch resistant coating material. Japan Kokai Tokkyo Koho JP 63 99,268 to Nishiuchi et al. also obtained an abrasion resistant and glossy coating material from a mixed system of alkali metal oxides. Japan Kokai Tokkyo Koho JP 64 01,769 to Yamada et al. used zirconium-containing organosiloxanes as hard coating materials which were based on a 1:1 molar ratio of Zr(OBu).sub.4 with Si(MeO).sub.3 H and ethylacetoacetate as the chelating agent. A pencil hardness value 4H was observed for the Yamada et al. system. Based on the same composition without Zr(OBu).sub.4, the pencil hardness value was reduced to only an HB value. The disclosure of these references describe coatings, which provide good adhesion of coatings on polymer. However, the disclosures are limited to inorganic systems with no functionalized organic reactants.
U.S. Pat. No. 4,929,278 to Ashley et al. also discloses various sol-gel derived coatings on plastics that are also inorganic in nature and do not contain functionalized organic reactants.
U.S. Pat. No. 4,746,366 to Philipp et al. describes the formation of scratch-resistant coatings that can be applied to plastic substrates, and which are formed by the hydrolytic polycondensation of at least one titanium or zirconium compound (such as tetraethyl titanate or tetrapropyl zirconate) and at least one organofunctional silane. In these systems, the organic moiety has a single silane functionality at one end that is capable of hydrolytic polycondensation with the titanium or zirconium compound.
U.S. Pat. No. 5,316,855 to Wilkes et al. also discloses a series of new high abrasion resistance coating materials. The materials are prepared utilizing organic/inorganic hybrid materials formed by co-condensing a metal alkoxide sol (e.g., silicon, aluminum, titanium, or zirconium metal alkoxide sol) with one or more bis(trialkoxysilane-containing) organic components or related functionalized species. The hybrid materials show optical clarity and improve the abrasion resistance of polymer substrates when applied as coatings and cured on the substrates. The coatings are applied to the substrate material, and are then dried and cured over a period of several hours.
An additional problem that must be addressed with respect to the abrasion resistant coatings is the susceptibility of the coatings to water damage. In particular, hot or boiling water can dramatically weaken the coating properties. The degradation caused by water results from the presence of water at the substrate/coating interface, which may weaken or completely break the coating/substrate bonding. The result can be delamination of the coating film from the substrate.
A further problem with conventional abrasion resistant coatings is that they are susceptible to degradation from exposure to ultraviolet radiation. In particular, most commercial organic polymers undergo chemical changes upon exposure to UV light (290 to 400 nm) because they possess chromophoric groups capable of absorbing UV light. This absorption of UV light by the polymers results in noticeable physical and chemical changes. Incorporation of a UV stabilizer into the coating system protects both the coating and the substrate from UV radiation. This leads to durability of the product.
However, there remains a need for improved abrasion resistant coatings for transparent polymeric material and metal surfaces. In particular, the need continues to exist for coatings that not only improve the 5 abrasion resistance of the underlying substrate, but that can be more easily and quickly applied in a commercial environment. Moreover, the need continues to exist for material coatings that can withstand more stringent conditions, such as conditions known as "hot-wet" conditions in which the coating is exposed to water at elevated temperatures.